Vertical Take Off and Landing (VTOL) aircraft have proven to be very useful transportation devices. In fact there are certain tasks, such as rough terrain rescue, where no other vehicle can approach the utility of a VTOL aircraft. If we are ever to realize the goal of universal personal aviation, easy-to-operate and inexpensive VTOL aircraft must be available.
The most common VTOL aircraft today is the single main rotor helicopter, usually equipped with a horizontal axis (yaw) tail rotor to counteract the torque produced by the main rotor shaft. Igor Sikorsky is generally credited with having invented this classic common helicopter. The first practical example, the Sikorsky model VS300, first flew in May 1940. Of all the VTOL aircraft based on conventional aircraft technology, the Sikorsky-inspired single main rotor, single tail rotor, or “classic helicopter” configuration has been the most widely adopted.
However, there are several pronounced drawbacks to the classic helicopter that many inventors have attempted to mitigate.
Classic helicopters are much more difficult to operate as compared to conventional aircraft. They require continuous, non-intuitive adjustments of power, pitch, roll, and anti-torque yaw forces.
Classic helicopters require a high degree of mechanical complexity in their control systems. Large drive and control forces must be applied to a large main rotor and simultaneously to a smaller tail rotor. These forces must be continuously variable, resulting in a myriad of rods, gears, bearings, and connection points. This complexity increases cost, making classic helicopters significantly more expensive to manufacture than conventional fixed-wing aircraft.
The main rotor of a classic helicopter must be made with enough mass so that it continues to rotate after an engine failure, allowing the pilot enough time to establish a windmill-style “glide” so the helicopter can be landed safely.
This procedure is referred to as “autorotation”, and is the only way a classic helicopter can survive an engine failure in flight. This high rotor mass (100 Kg or more per blade), coupled with the fact that lateral thrust is obtained from the main rotor by continuously varying the pitch of the rotor blades as they traverse the arc of rotation (cyclic pitch), means the control forces required are very high. This places considerable stresses on control linkages, which in turn, leads to high maintenance costs and failure potential. This high blade mass also represents a greater impact energy hazard to ground personnel and structures during takeoff and landing.
Classic helicopters have a high degree of minimum complexity. In other words, virtually every component represents a single point of failure. If any part of a classic helicopter fails, the helicopter stops flying. There is no fault tolerance. In order to insure a modicum of reliability, even the smallest part in a classic helicopter must be manufactured from the finest materials to exacting tolerances. Again, this makes classic helicopters very expensive to produce.
Classic helicopters place very high stresses on single parts, and there is no redundancy. For example, a single main rotor shaft carries the entire weight of the fuselage and payload. Because of this, critical parts tend to wear out relatively quickly. Highly trained personnel must perform complex tasks according to rigorous maintenance schedules to insure a reasonable degree of safety. This significantly increases the operating costs above that required for conventional aircraft
In short, the trade offs for the immense utility of the classic helicopter have been high procurement and maintenance costs, coupled with a very high skill level demand upon the pilot.
Since the original Sikorsky flew, many attempts to improve upon the classic helicopter concept have been tried. Virtually all have failed, largely due to four main factors.
Many prior inventions attempt to mitigate high pilot skill requirements with higher drive unit mechanical complexity, leading to an increased failure probability and a higher cost.
Many prior inventions embody improper aerodynamic considerations, including rotor disk loading factors that are too high to be efficient or practical.
Prior inventions attempt to combine the utility of VTOL aircraft with the high forward speed attributes of fixed-wing aircraft, in the misguided assumption that the main drawback to the classic helicopter is a low forward speed capability. In the end, these manifestations of multiple articulating engine and/or lifting surfaces actually increase complexity and risk of catastrophic failure. Witness the fact that no tilt engine or tilt wing has ever achieved any commercial success except in esoteric military applications.
Some modest design variations on the classic helicopter configuration have enjoyed limited success, mainly due to elimination of the tail rotor component. Coaxial or “over under” main rotor designs have been employed to achieve a torque-effect cancellation (prior art Hiller 1944, Norris, U.S. Pat. No. 6,460,802, and others). VTOL aircraft employing multiple rotors predate Sikorsky, with quad rotors (prior art G. de Bothezat, 1922) or Side-by-side (Weir-Cierva, 1936; Madet, U.S. Pat. No. 3,889,902).
However, while some gains may have been made in reduction of piloting skill requirements, this came at the expense of multiplying the main rotor system complexity and associated costs.
Any single rotor or propeller VTOL aircraft must have some means of counteracting the torque produced by the mechanical twisting of the rotor shaft. This requires incorporating a lateral (yaw force-producing) thrusting means somewhere in the airframe. The now famous NOTAR (VanHorn, U.S. Pat. No. 4,948,068) system eliminates the classic helicopter tail rotor by deflecting some downwash from the single main rotor (combined with turbine exhaust gasses), but this configuration still requires a much more complex mechanical control system than a conventional fixed-wing aircraft. Virtually all other torque compensation systems require an additional thrusting power source directed laterally, increasing complexity and costs.
It is universally recognized that incorporating multiple, redundant power plants into an aircraft can improve reliability. While this does add complexity, it is more than compensated for by the added safety and reliability.
Witness the fact that virtually every commercial passenger aircraft is equipped with two or more engines. However, there have been scant few examples of multi-engine, multi-rotor helicopters in commercial use, and virtually none of those can remain in operation if one or more rotors fail.
An ideal solution to the problems associated with Classic Helicopters would to be VTOL aircraft with a redundant number of vertical thrusting propellers or rotors, each offsetting, dispersing, or canceling the torque of the other(s), while maintaining a simple, preferably non-mechanical, control system. This precludes the use of propeller or rotor “tilting” technology or elaborate pitch, length, or other rotor axis articulation means to accomplish the goal.
The avoidance of mechanical thrust vectoring means eliminating potential failure modes. For example, if a tilting drive unit failed to tilt properly, and at the appropriate time, a dangerously unstable flight condition would result.
What is needed is a multiple rotor VTOL aircraft with an electrical or “fly by wire” control system. Some inventors have recognized the promise afforded by the advent of modern power electronic control devices. There have been a few attempts to incorporate electrical control means to multi-rotor VTOL aircraft, but they have not been successful, mainly due to aerodynamic inefficiency. These newer electric power and/or control designs typically reflect a desire to have an aircraft with a “flying saucers” appearance, rather than addressing fundamental aerodynamic problems.
Milde, U.S. Pat. No. 6,179,247 and Bucher, U.S. Pat. No. 6,254,032 reveal attempts to utilize multiple engines and multiple rotors for VTOL aircraft, but neither seems to incorporate basic aerodynamic disk loading considerations. The aircraft weight versus thrust disk area ratio is simply too high for them to exhibit hover efficiencies anywhere close to a classic helicopter, if they could be made to fly at all. This high disk area loading is largely due to the fact that the fuselage is located in the rotor plane of rotation itself, subtracting from the available disk area.
This contrasts with a conventional Classic helicopter, where the main fuselage only minimally subtracts from the available rotor disk area and does so at the root of the rotor disk.
Wagner, et al. (USP Ap. 2003/0085319) depicts an attempt to mitigate these aerodynamic shortcomings of a multi-engine propeller VTOL by incorporating fixed wings. Still, when compared to the rotor disk area of the ubiquitous Classic helicopter, this craft could not operate in hover in an efficient or fault-tolerant manner. Also, the thrust units utilize a “tilt” feature that adds costly complexity, and represents an added hazardous point of failure.
The Wobben patent (U.S. Pat. No. 6,293,491) teaches multiple electric motors, but also uses fixed wings, as well as dissimilar thrusting and lifting rotors. This approach adds complexity and aerodynamic inefficiencies in another misguided attempt to combine fixed-wing aircraft attributes with standard helicopter maneuverability. It would seem unlikely that the practice of the Wobben patent could result in an efficient or cost-effective aircraft.
The Wobben approach also does not include any redundant pairing of thrust units to maintain a substantial aerodynamic consistency in the event of a thrust unit failure. It also does not use the multiple electric lifting thrust units for lateral thrusting, relying instead on standard horizontal thrusting in combination with regular aerodynamic control surfaces. This adds unnecessary complexity and cost.